It depends on the type of fish you plan to keep. You have mollies, correct? I would keep 2 or three trios (1 males, 2 females is a trio) for now. Add lots of bushy plants, your tank will be full in no time.

i fyou plan to add other types of fish, make sure they are compatible and have similar water needs (tropical, preferably brackish). If your tank is not yet cycled, let it cycle before adding more fish.

After it cycles, cories could be a really fun bottom feeder to add. They like schools of four or more. In a 55, I would add probably 7-8. They are very interesting and fun to watch. Funny little personalities.

first you say which type of fish you wish to keep in your aquarium. then it will be easier for us to tell about how many fish you can keep....also take a look about the compatible fish chart from this link... http://www.aquariumlife.net/fishcomp.asp

you could have as much filtration as you want but filtration wouldn't be the problem, two fully grown oscars would simply not have enough space in a 55 gallon. yes they could live in a 55 but they would have very little room between 2 of them, which can lead to aggression. i would only keep 1 fully grown oscar in a 55 gallon, you could however have a few juvinile oscars but you would have to remove some over time as they mature and grow (which oscars do at a fast rate lol)